Method for the combustion of fuel

ABSTRACT

The present invention relates to a process for the combustion of fuel ( 5 ) supplied to the combustion zone ( 2 ) of a burner ( 1 ), using oxygen or a mixture of oxygen and carbon dioxide. The invention further relates to a burner for carrying out the process of the invention. To provide a process allowing stable combustion of the fuel using oxygen or a mixture of oxygen and carbon dioxide at a carbon content of less than 21% by volume, the invention proposes a cyclic flow ( 8 ) being created in the combustion zone ( 2 ) through which a homogeneous mixing of hot combustion products with the fuel ( 5 ) and the oxygen on the one side and with the fuel ( 5 ) and the mixture of oxygen and carbon dioxide on the other side takes place in the combustion zone ( 2 ).

The present invention relates to a process for the combustion of fuelsupplied to the combustion zone of a burner, using oxygen or a mixtureof oxygen and carbon dioxide. The invention further relates to a burnerfor carrying out the process according to the invention.

Such processes have become increasingly important in the past years,because this kind of combustion allows a drastic reduction of thepollutant emission compared to the combustion of fuel using air.

Conventional combustion processes, in which for example fossil fuels areburnt, mostly take place under the use of air, i.e. air is supplied tothe combustion. Considering however, that air has a very high nitrogencontent of approx 80%, such combustion processes produce nitrogen oxideswhich are an extreme burden to the environment. Further, during thecombustion of fossil fuels, carbon dioxide is produced which is alsopollutant, because it intensifies the greenhouse effect. Thus an attemptmay be made to separate the nitrogen and the carbon dioxide which arepresent in the flue gas as a mixture, so that the carbon dioxide can besubsequently stored and eventually used for other purposes. But thisseparation is very expensive, because at the end of the process a fluegas mixture is present which has carbon dioxide content of approx 20%and a nitrogen content of approx 80%.

For this reason, oxygen or a mixture of carbon dioxide and oxygeninstead of air were supplied to the combustion, resulting in that at theend of the process almost pure carbon dioxide is present which can bestored afterwards. Another advantage is that no harmful nitrogen oxidesare emitted during such a combustion process.

If the combustion is performed using a mixture of oxygen and carbondioxide, it turned out to be beneficial if the flue gas produced duringthe combustion is removed from the combustion chamber and issubsequently recycled again to the combustion chamber, wherein thisrecycled flue gas has a high carbon dioxide content when fossil fuelsare burnt. Oxygen which is required for the combustion has to beproduced externally and mixed with the recycled flue gas.

Accordingly, in the above-described combustion of fuel usingoxygen-enriched flue gas, the nitrogen originating from the air issubstantially replaced by carbon dioxide. But as carbon dioxide has ahigher thermal capacity than nitrogen, more heat is extracted from thecombustion than when air is used. This causes the temperature in thecombustion zone to drop, so that the combustion takes place under nooptimum conditions. For this reason, the combustion is preformed at alevel of oxygen (>21%) which is increased compared to air, whereby thecombustion is stabilized. On the other hand, the costs aresimultaneously increased, because the pure oxygen must be produced inadvance.

For the reasons as mentioned, such combustion processes, also calledoxy-fuel processes, are much friendlier to the environment than theprevious combustion in which air is supplied.

In the described combustion processes the carbon dioxide noticeablyreacts with carbon at a temperature from approx 700° C., whereby carbonmonoxide is produced. The corresponding reaction equation is as follows:

CO₂+C→2 CO

This reaction produces a duplication of the volume, since two mol ofcarbon dioxide are produced from one mol of carbon monoxide. Thisexpansion of the volume causes higher flow velocities which negativelyinfluence the stability of the combustion reaction, hence the combustionprocess. Additionally, in the above endothermic reaction heat isdissipated, which fact also destabilizes the combustion process. Due tothe reduction of the combustion temperature the above reaction does notoccur any longer, resulting in that the accompanying duplication of thevolume does not take place either. This causes the combustiontemperature to increase again to a level where the reductivetransformation of carbon dioxide into carbon monoxide is permitted. Thecombustion process is weakened in the end by the mentioned feedbackprocesses, since there are no constant conditions for the combustion.For this reason, for stabilizing the combustion process, oxygen at avolume of at least 21% has been supplied to the combustion zone up topresent, whereby the temperature of the combustion process could bemaintained relatively constant. This use of oxygen adds to the cost of acorresponding combustion.

In view of the above, it is an object of the present invention toprovide a process for the combustion of fuel in a burner, in whichprocess a combustion of the fuel can be stably performed using oxygen ora mixture of oxygen and carbon dioxide at an oxygen content ofpreferably less than 21% by volume.

According to the invention, this object is achieved by the process forthe combustion of fuel supplied to a combustion zone of a burner usingoxygen or a mixture of oxygen and carbon dioxide. The fuel can be fossilfuel such as coal or the like which is fed in a suitable form to thecombustion zone. The mixture of oxygen and carbon dioxide is preferablyformed from oxygen-enriched flue gas that is removed from the combustionchamber and subsequently recycled again to the combustion chamber.

According to the invention, a cyclic flow is created in the combustionzone causing a homogeneous mixing of hot combustion products with thefuel and the oxygen or with the fuel and the mixture of oxygen andcarbon dioxide in the combustion zone. This cyclic flow transports heatand combustion products to the combustion zone of the burner, thusstabilizing the combustion in the combustion zone of the burner, becausethe above-described reduction of heat caused by the endothermic reactionis compensated by maintaining the hot combustion products in thecombustion zone. This does not comprise conditions like those met in theFLOX technology in which air is used as oxidant. To create a cyclic flowsufficient for stabilizing the combustion, parameters such as the supplyrates of the oxygen or the mixture of oxygen and carbon dioxide and ofthe fuel as well as the physical structure of the burner must beconsidered. A suitable burner includes for instance in the supply areaof the materials flow separation edges capable of causing swirling ofthe materials which are supplied. Further, in the combustion zone of theburner an appropriately designed space must be available enabling aturbulent flow to create a cyclic flow in the form of a stronglyrecycling flow. By this stabilization of the combustion in thecombustion zone of a burner by means of a strongly recycling flow, theoxygen content in the combustion zone can be reduced, since thecombustion needs not be stabilized alone by supplying oxygen.Accordingly, it is possible to keep the oxygen content in the combustionzone below 21% by volume and yet achieve a stable combustion process.

An advantageous embodiment of the invention provides that the cyclicflow is created in the form of a strongly reduced flow substantially inthe entire combustion zone of the burner, whereby the combustion processis maximally stabilized and whereby the oxygen content in the combustionzone can be reduced as far as possible. Preferably, the stronglyrecycling flow is created in at least 50% of the combustion zone.Preferably, the recycling flow is created in at least 80% of thecombustion zone. Particularly preferably, the recycling flow is createdin at least 90% of the combustion zone.

A further advantageous embodiment of the invention provides that fluegas produced during the combustion is discharged from the combustionzone. This flue gas has a high carbon dioxide content and can be storedafter its discharge, so that the carbon dioxide emission during thecombustion in accordance with the invention is reduced as far aspossible.

A further advantageous embodiment of the invention provides that theflue gas is enriched with oxygen and thereafter supplied to thecombustion zone. By the enrichment of the flue gas with oxygen prior tobeing supplied to the combustion zone, a homogeneous mixing of flue gasand oxygen is achieved, which has a favorable effect on the combustionprocess. Further, by supplying the hot flue gas, heat is supplied to thecombustion process in the combustion zone, which also has a positiveeffect on the combustion process.

A further advantageous embodiment of the invention provides that thefuel is supplied to the combustion zone together with the oxygen and/orflue gas, wherein the flue gas may be a mixture of oxygen and carbondioxide. The way which is selected for the supply depends on therespective conditions, e.g. on the design of the burner or on otherparameters of the combustion, so that the process of the invention canbe suitably adapted for optimum combustion.

A further advantageous embodiment of the invention provides that thefuel, the oxygen and/or the flue gas are supplied to the combustion zonefrom different directions or from the same direction. Accordingly,depending on the design of the burner, the creation of a cyclic flow inthe form of a strongly recycling flow can be promoted by the variantselected for supplying the materials to the combustion zone. Forexample, a supply of the materials in opposite directions can result ina strongly recycling flow being created.

Further, an advantageous embodiment of the invention provides that thecomponents of the flue gases are completely burnt in the combustion zoneor are removed from the combustion zone for use thereof for otherpurposes outside the combustion zone. For instance, a suitable selectionof the parameters of the combustion allows the targeted production ofcarbon monoxide that can be discharged from the burner and utilized as astarting material for particular chemical compositions.

Concerning the burner, the afore-mentioned object of the presentinvention is achieved by the combustion zone of the burner includingfluid-mechanical installations, so that the burner is suitable forcreating the cyclic flow in the form of a strongly recycling flow, thusallowing the use of the process according to the invention.

An advantageous embodiment of the invention provides that the burnerincludes several supply channels which are part of the fluid-mechanicalinstallations of the burner. Through these supply channels the fuel, theoxygen, the mixture of oxygen and carbon dioxide, and the flue gas arefed to the combustion zone of the burner, and it is possible to supplyseveral ones of the mentioned materials through a single channel,whereby uniform mixing of these materials takes place before thematerials are supplied to the combustion zone, which may have abeneficial effect on the combustion. Supplying the respective materialsthrough a respective separate channel is also possible, should this bepreferred. The supply channels can be so arranged that the materials aresupplied from different directions or from the same direction. Supplyingthe materials from opposite directions for instance promotes andenhances the creation of a cyclic flow, which has a positive effect onthe capability of reducing the amount of oxygen needed during thecombustion.

A further advantageous embodiment of the invention provides that theburner includes a swirling installation. Through this swirlinginstallation, which may be designed as fan or the like, the creation ofa cyclic flow in the form of a strongly recycling flow can be promoted,thus providing the described advantage of oxygen reduction.

Further advantages and features of the present invention will bedescribed in the following with reference to the attached drawingswherein it is shown by

FIG. 1 a schematic illustration of one embodiment for a burner accordingto the invention in which fuel is burnt in the combustion zone inaccordance with the process of the invention.

FIG. 1 shows a burner 1 with a combustion zone 2, and two supplychannels 3 and 4 provided in the burner 1, which are arranged inparallel and through which a carbon-containing fuel 5 is fed to thecombustion zone 2 of the burner 1 together with oxygen-enriched flue gas6 or through which oxygen-enriched flue gas 7 alone is fed to thecombustion zone 2 of the burner 1. The flow velocities of the materials6 and 7 should be so selected that a difference between the flowvelocities of the materials 6 and 7 causes a cyclic flow 8 in the formof a strongly recycling flow, as indicated by the arrows in FIG. 1,being created in the combustion zone 2. To this end, the gaseous fluid7, which enters through the supply channel 4 shown on the right side inFIG. 1, has a higher flow velocity than the gaseous fluid 6 which entersthrough the supply channel 3 shown on the left side and which hasadmixed to it powdery fuel 5. The left half of the cyclic flow 8automatically supplies the combustion zone 2 with hot combustionproducts, whereby the combustion in the combustion zone 2 can bestabilized. Of course, it is also conceivable for the supply channels 3and 4 being arranged in a different manner or being provided in adifferent number, which is particularly suited for creating a cyclicflow in the form of a strongly recycling flow 8 that fills out theentire combustion zone 2, if possible. Further, in FIG. 1 a flue gasdischarge and supply passage 9 is drawn, through which the flue gas isdischarged from the combustion zone 2 and, as indicated by arrow 10, issupplied again to the inflowing fluids 6 and 7 prior to entering thecombustion zone 2 of the burner 1.

The above described embodiment is intended for the purpose ofillustration only, without limiting the invention.

LIST OF REFERENCE NUMBERS

1 burner

2 combustion zone

3 supply channel

4 supply channel

5 fuel

6 oxygen and flue gas

7 oxygen and flue gas

8 cyclic flow

9 discharge and supply passage

10 arrow

1.-10. (canceled)
 11. A process for the combustion of fuel supplied tothe combustion zone of a burner using oxygen or a mixture of oxygen andcarbon dioxide, wherein in the combustion zone a strongly recyclingcyclic flow is created through which heat and combustion products aretransferred to the combustion zone, whereby the combustion in thecombustion zone is stabilized.
 12. The process according to claim 11,wherein the cyclic flow is created substantially in the entirecombustion zone.
 13. The process according to claim 11, wherein thefuel, the oxygen and/or the flue gas are supplied to the combustion zonefrom different directions or from the same direction.
 14. The processaccording to claim 11, wherein the components of the flue gases arecompletely burnt in the combustion zone or are partly removed from thecombustion zone to be used for different purposes outside the combustionzone.
 15. A burner comprising a combustion zone for the combustion offuel supplied to the combustion zone, using oxygen or a mixture ofoxygen and carbon dioxide, wherein the combustion zone of the burnerincludes fluid-mechanical installations for creating a stronglyrecycling cyclic flow in the combustion zone, through which heat andcombustion products are transferred to the combustion zone.
 16. Theburner according to claim 15, comprising several supply channels. 17.The burner according to claim 15, comprising a swirling device.
 18. Theburner according to claim 16, comprising a swirling device.